1. Field of the Invention
This invention generally relates to electrical drivers for a matrix of current driven devices, and more particularly to methods and apparatus for avoiding droop of precharged column voltage in such devices.
2. Description of the Related Art
There is a great deal of interest in “flat panel” displays, particularly for small to midsized displays, such as may be used in laptop computers, cell phones, and personal digital assistants. Liquid crystal displays (LCDs) are a well-known example of such flat panel video displays, and employ a matrix of “pixels” which selectably block or transmit light. LCDs do not provide their own light; rather, the light is provided from an independent source. Moreover, LCDs are operated by an applied voltage, rather than by current. Luminescent displays are an alternative to LCD displays. Luminescent displays produce their own light, and hence do not require an independent light source. They typically include a matrix of elements which luminesce when excited by current flow. A common luminescent device for such displays is a light emitting diode (LED).
LED arrays produce their own light in response to current flowing through the individual elements of the array. The current flow may be induced by either a voltage source or a current source. A variety of different LED-like luminescent sources have been used for such displays. The embodiments described herein utilize organic electroluminescent materials in OLEDs (organic light emitting diodes), which include polymer OLEDs (PLEDs) and small-molecule OLEDs, each of which is distinguished by the molecular structure of their color and light producing material as well as by their manufacturing processes. Electrically, these devices look like diodes with forward “on” voltage drops ranging from 2 volts (V) to 20 V depending on the type of OLED material used, the OLED aging, the magnitude of current flowing through the device, temperature, and other parameters. Unlike LCDs, OLEDs are current driven devices; however, they may be similarly arranged in a 2 dimensional array (matrix) of elements to form a display.
OLED displays can be either passive-matrix or active-matrix. Active-matrix OLED displays use current control circuits integrated within the display itself, with one control circuit corresponding to each individual element on the substrate, to create high-resolution color graphics with a high refresh rate. Passive-matrix OLED displays are easier to build than active-matrix displays, because their current control circuitry is implemented external to the display. This allows the display manufacturing process to be significantly simplified. Whether internal or external, the control circuitry of OLED displays requires various complicated schemes relating to the supply and timing of different voltages and currents.
In a typical display matrix, OLEDs require a minimum voltage level in order to illuminate. Because providing such minimum voltage to an OLED using only a current source is typically slow, display matrix technology implements the use of a voltage source to precharge OLEDs before the desired illumination time of the OLEDs. Thus, when a current source is applied to illuminate the OLEDs, it is desirable to have the minimum voltage level on the OLEDs to immediately illuminate the OLEDs. However, even when the voltage source is used to precharge the OLEDs, there is an undesirable drop in voltage across the OLED when the current source is applied. This drop may cause undesirable delays in illumination and/or improper illumination. Thus, a system and method for compensating for the delays in illumination and/or improper illumination is needed.